1. Field of the Invention
The present invention relates to rotary position sensors, and more particularly, to encoders therefor which provide digital representations of angular position, free of errors induced by non-rotational motion and component strain.
2. Description of the Prior Art
Gimbals for pointing transmitting and receiving elements in tracking and communication systems, such as those employing lasers, must be positionable with extremely high accuracies. Angular position of these gimbals, from a fixed reference point, is determined through rotary position encoders which convert angular location to coded electrical signals that are utilized to determine present angular position relative to a desired end location. To achieve the end position accuracy required for laser and other satellite communication systems, rotary position encoders first convert the angular position to a coded optical signal. This coded optical signal is utilized to generate the electrical position signal. Optical encoders of the prior art need window sizes as small as 100 microinches to establish the optical resolution required to achieve the desired angular positioning accuracies. This necessitates synthesizing schemes which exhibit great complexity, excessive size, and high cost. Various investigations of novel encoding methods have been undertaken to overcome these deficiences. Complete success, however, has eluded the investigators because of the significant error contribution provided by the uncertainty of the location of the center of rotation of the disc. Though this uncertainty may be reduced to some extent by design and assembly precision, such uncertainty reduction increases the complexity and cost of the system, thus negating the initial achievements. Additionally, errors related to temperature and life induced strain are not always reducible and further limit the accuracies obtainable with these innovations.
Photoengraving techniques, though providing significant decreases in the window size, have not resulted in direct improvement of encoder resolution since non-rotational movements of the code wheel are mixed with the rotational movement of the window. Additionally, the reduced window size reduces the travel distance to be sensed by the pick-off, thus requiring higher resolution photodetectors which add significantly to the cost.
To achieve the desired accuracies with the prior art systems, it is not uncommon to have to repeatedly assemble and disassemble an encoder until an acceptable set of geometrics has been obtained. It is also not uncommon to temperature cycle an encoder during various subassembly stages to set the disc geometrically. Should this temperature cycling result in a set position which is unacceptable the unit is disassembled, reassembled and the process is performed again until satisfactory results are obtained. These problems, not only drive the manufacturing cost up, but combine to provide a somewhat fragile end product from an environmental and equipment life standpoint.